Histone dynamics during transcription: exchange of H2A/H2B dimers and H3/H4 tetramers during pol II elongation.

Chromatin within eukaryotic cell nuclei accommodates many complex activities that require at least partial disassembly and reassembly of nucleosomes. This disassembly/reassembly is thought to be somewhat localized when associated with processes such as site-specific DNA repair but likely occurs over extended regions during processive processes such as DNA replication or transcription. Here we review data addressing the effect of transcription elongation on nucleosome disassembly/reassembly, specifically focusing on the issue of transcription-dependent exchange of H2A/H2B dimers and H3/H4 tetramers. We suggest a model whereby passage of a polymerase through a nucleosome induces displacement of H2A/H2B dimers with a much higher probability than displacement of H3/H4 tetramers such that the extent of tetramer replacement is relatively low and proportional to polymerase density on any particular gene.